dt-bindings: display: rockchip-dsi: document external phys

Some dw-mipi-dsi instances in Rockchip SoCs use external dphys.
In these cases the needs clock will also be generated externally
so these don't need the ref-clock as well.

changes in v5:
- rebased on top of 5.5-rc1
- merged with dsi timing change to prevent ordering conflicts

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191209143130.4553-4-heiko@sntech.de